Fibers having combined two types of synthetic resin having different properties are known as composite fibers which are chemical fibers having crimpability and a fibril structure. One prior art method for preparing such composite fibers involves the steps of stretching and then slitting a composite synthetic resin film of two layer structure consisting of two materials having different properties, for example, two layers of polypropylene and polyethylene, thereby forming stretched tapes and fibrillating the stretched tapes into split fibers as disclosed in Japanese Patent Application Kokai No. 149905/1987.
Split fibers or yarns obtained by fibrillation of prior art known composite synthetic resin films, however, are undesirably susceptible to delamination while composite synthetic resin films are liable to layer separation during stretching. For example, composite synthetic resin films consisting of polypropylene and polyethylene layers suffered from the powdering problem that polyethylene is separated away upon fibrillation.
Some of the present inventors proposed in Japanese Patent Application No. 48223/1988 filed Mar. 1, 1988 (Japanese Patent Application Kokai No. 221507/1989), a method for preparing split fibers having improved crimpability and a fibril structure using a composite synthetic resin film having improved interlaminar bonding and stretchability while minimizing powdering during fibrillation as well as an integrated split fiber article of network structure formed from such split fibers. More particularly, the method for preparing split fibers includes the steps of: slitting and then stretching or stretching and then slitting a composite synthetic resin film having at least two layers, thereby forming stretched tapes, and fibrillating the stretched tapes into split fibers, characterized in that the composite synthetic resin film is a composite synthetic resin film in which one layer is a polypropylene layer formed of a mixture of 70 to 95% by weight of a polypropylene having a melt index of 0.5 to 10 and 30 to 5% by weight of a polyethylene having a melt index of 0.5 to 20 and the other layer is a polyethylene layer formed of a mixture of 70 to 95% by weight of a polyethylene having a melt index of 0.5 to 20 and 30 to 5% by weight of a polypropylene having a melt index of 0.5 to 10.
Also proposed in the last application is a method for preparing an integrated split fiber article, comprising the steps of: slitting and then stretching or stretching and then slitting a composite synthetic resin film having at least two layers, thereby forming stretched tapes, fibrillating the stretched tapes into split fibers, mixing the resultant split fibers alone or with plant fibrous material, and heating the mixture at a temperature between the melting points of the polyethylene and the polypropylene, thereby integrating together the split fibers with each other or with the plant fibrous material.
In mixing such split fibers alone or with plant fibers as typified by pulp and thermally fusing the split fibers together or with the plant fibers, especially under a substantially no pressure condition, the bond strength between split fibers or between split fibers and plant fibers is not necessarily sufficient because the polyethylene of the polyethylene layer forming the split fibers has poor melt flow and is susceptible to thermal shrinkage. Bond strength is low particularly when split fibers are integrated with plant fibers. In addition, the integrated split fiber article itself undergoes thermal shrinkage, leaving a room for improving dimensional stability.